The disclosure follows angry demands by lawmakers earlier in the day for congressional inquiries into whether the monitoring by the highly secretive National Security Agency violated civil liberties.

"There is no doubt that this is inappropriate," declared Republican Sen. Arlen Specter of Pennsylvania, chairman of the Senate Judiciary Committee. He promised hearings early next year.

Bush on Friday refused to discuss whether he had authorized such domestic spying without obtaining warrants from a court, saying that to comment would tie his hands in fighting terrorists.

In a broad defense of the program put forward hours later, however, a senior intelligence official told The Associated Press that the eavesdropping was narrowly designed to go after possible terrorist threats in the United States.

The official said that, since October 2001, the program has been renewed more than three dozen times. Each time, the White House counsel and the attorney general certified the lawfulness of the program, the official said. Bush then signed the authorizations.

During the reviews, government officials have also provided a fresh assessment of the terrorist threat, showing that there is a catastrophic risk to the country or government, the official said.

"Only if those conditions apply do we even begin to think about this," he said. The official spoke on condition of anonymity because of the classified nature of the intelligence operation.

"The president has authorized NSA to fully use its resources - let me underscore this now - consistent with U.S. law and the Constitution to defend the United States and its citizens," the official said, adding that congressional leaders have also been briefed more than a dozen times.

Senior administration officials asserted the president would do everything in his power to protect the American people while safeguarding civil liberties.

"I will make this point," Bush said in an interview with "The NewsHour With Jim Lehrer." "That whatever I do to protect the American people - and I have an obligation to do so - that we will uphold the law, and decisions made are made understanding we have an obligation to protect the civil liberties of the American people."

The surveillance, disclosed in Friday's New York Times, is said to allow the agency to monitor international calls and e-mail messages of people inside the United States. But the paper said the agency would still seek warrants to snoop on purely domestic communications - for example, Americans' calls between New York and California.

"I want to know precisely what they did," Specter said. "How NSA utilized their technical equipment, whose conversations they overheard, how many conversations they overheard, what they did with the material, what purported justification there was."

Sen. Russ Feingold, D-Wis., a member of the Judiciary Committee, said, "This shocking revelation ought to send a chill down the spine of every American."

Vice President Dick Cheney and Bush chief of staff Andrew Card went to the Capitol Friday to meet with congressional leaders and the top members of the intelligence committees, who are often briefed on spy agencies' most classified programs. Members and their aides would not discuss the subject of the closed sessions.

The intelligence official would not provide details on the operations or examples of success stories. He said senior national security officials are trying to fix problems raised by the Sept. 11 commission, which found that two of the suicide hijackers were communicating from San Diego with al-Qaida operatives overseas.

"We didn't know who they were until it was too late," the official said.

Some intelligence experts who believe in broad presidential power argued that Bush would have the authority to order these searches without warrants under the Constitution.

In a case unrelated to the NSA's domestic eavesdropping, the administration has argued that the president has vast authority to order intelligence surveillance without warrants "of foreign powers or their agents."

"Congress cannot by statute extinguish that constitutional authority," the Justice Department said in a 2002 legal filing with the Foreign Intelligence Surveillance Court of Review.

Other intelligence veterans found difficulty with the program in light of the 1978 Foreign Intelligence Surveillance Act, passed after the intelligence community came under fire for spying on Americans. That law gives government - with approval from a secretive U.S. court - the authority to conduct covert wiretaps and surveillance of suspected terrorists and spies.

In a written statement, NSA spokesman Don Weber said the agency would not provide any information on the reported surveillance program. "We do not discuss actual or alleged operational issues," he said.

Elizabeth Rindskopf Parker, former NSA general counsel, said it was troubling that such a change would have been made by executive order, even if it turns out to be within the law.

Parker, who has no direct knowledge of the program, said the effect could be corrosive. "There are programs that do push the edge, and would be appropriate, but will be thrown out," she said.

Prior to 9/11, the NSA typically limited its domestic surveillance activities to foreign embassies and missions - and obtained court orders for such investigations. Much of its work was overseas, where thousands of people with suspected terrorist ties or other valuable intelligence may be monitored.

The report surfaced as the administration and its GOP allies on Capitol Hill were fighting to save provisions of the expiring USA Patriot Act that they believe are key tools in the fight against terrorism. An attempt to rescue the approach favored by the White House and Republicans failed on a procedural vote.